ON THE THEORY OF SOLUTION. 333 
that the ancient mystery of electrical fishes will find its solution on these 
lines. 
Referring to the discussion Professor OstWALD said :—Professor Fitz- 
gerald has asked why the ions, when they are free, do not separate by 
diffusion. The answer is that they do. If we have a solution of HCl, 
for example, consisting to a great extent of H and Cl ions, in contact 
with pure water, the H ions, moving much faster than the Cl ions, take 
the lead in wandering into the water. But a separation of electricity 
hereby takes place, and every ion being charged with a great amount of 
either positive or negative electricity, the electrostatic forces resulting 
from the initial separation soon prevent further separation. Therefore 
water must take a positive potential against a solution of hydrogen 
chloride, and in general water must show against every electrolytic 
solution the potential of the faster ion. 
These considerations, which lead to the whole theory of the potential 
differences between electrolytes, were first developed by W. Nernst 
(‘Zeitsch. f. phys. Chem.’ ii. 613, and iv. 129), who has confirmed them by 
various experiments ; and further by M. Planck (Wied. ‘ Ann.’ xl. 561). 
As far as Iam aware, no theory of fluid-cells (Fliissigkeitsketten) had 
hitherto existed, and the possibility of developing one consistent with 
experiment from the principles first stated by Arrhenius is strong evidence 
in favour of his views. 
Secondly, Professor Fitzgerald seeks for the source of energy required 
for the separation of, e.g., Cl and H by dissolving HCl in water. This 
question is in accordance with the widely-spread assumption that a great 
expenditure of work must be done to effect this separation. As a great 
amount of heat is developed by forming HCl from its elements it seems 
evident that the same amount of energy must be restored to the elements 
in separating them. This is quite true if common hydrogen and chlorine 
were formed, but the ions H and Cl, existing in the aqueous solution of 
hydrogen chloride, are by no means identical with the so-called free 
elements. To use a word to which chemists are accustomed, the ions H 
and Cl are allotropic forms of these elements, similar to yellow and red 
phosphorus, and contain very different amounts of energy from those 
which they contain in their common state of hydrogen and chlorine gases. 
Therefore it is impossible to say anything @ priori about the evolution or 
absorption of energy connected with the change from HCl gas to positively 
charged H ions and negatively charged Cl ions; we must interrogate 
facts ; and these teach us that the ions generally contain much less energy 
than the elements in the common state, and therefore a great amount of 
energy is not called for in the transformation of, e.g., HCl into the free 
ions H and Cl. 
The elements in the state of ions being charged with great amounts 
of electricity, the very different tendency of the elements to assume the 
state of ions can be conveniently called their different affinity for elec- 
tricity. This expression is of course only a fagon de parler, but it gives 
a good description of the behaviour of the elements. The action, for 
example, of zinc on cupric sulphate solution, containing the ions Cu and 
SO,, depends on the greater tendency of the zinc to form ions; therefore 
the zine tears the positive electricity necessary for its existence as an ion 
from the copper ions, and deposits the latter as unelectrical, i.e. common 
metallic copper. The SO, ions, being no closer connected with the zinc 
